During the past 14 years, Petroleos Mexicanos has carried on an extensive drilling program. This has been principally concentrated along the Frio-Vicksburg trend. The Frio has been the most prolific producing formation in northeastern Mexico. Of its three facies, non-marine, brackish, and marine, the first named is the most productive.

Locally overlying the Frio, both on the surface and in the subsurface, is the Norma

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conglomerate. It is a large fan deposit, which may be correlative to the Anahuac formation, or the Soledad formation.

Unconformably above the Norma, or the Frio, is the Anahuac formation. The three foramineferal zones, Discorbis, Heterostegina, and Marginulina, are recognized in northeastern Mexico. All are productive.

The most important fields discovered in the Frio-Vicksburg trend are Brasil, Reynosa, Cano, and Trevino, where development drilling is still going on. The 18 de Marzo field is important as a gas producer from the Marginulina zone of the Anahuac, and the Trevino field produces from the Heterostegina zone.

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